The bluntnose sixgill shark (Hexanchus griseus) is a large, deepwater, cosmopolitan species whose range
includes the Strait of Georgia, British Columbia. Although this species is probably the top predator on most of the
world’s continental shelves, there is little information on its population biology or behaviour. At a unique site off Flora
Islets, in the Strait of Georgia, a number of H. griseus individuals show unexpected diurnal activity on a shallow reef.
Regular movements of these sharks along the base of the reef wall allowed us to use an automated, time-lapse video
system to make the first extensive observations of their behaviour between June 2001 and July 2002. Sixgill shark frequency
(no. of sharks observed/h) increased in June and reached a peak between mid-June and mid-July. Sixgill sharks were not
observed between October and May. Sixgill shark frequency was highest in the afternoon and the sharks tended to move as
individuals rather than in groups. The observations suggest that this shallow-water activity is not related to either reproduction
or feeding and its purpose remains unclear.
We return to the Pacific Northwest with this 2003 paper from renowned diver and sixgill enthusiast Zielinski!
This study took place on a shallow reef of the Flora Islet from June 2001 to July 2002. A time-lapse camera was fixed to the reef wall 40m down and recorded an image every 9s. By doing so, they were able to get 240h of data on a 4h footage tape (yes, tape! Like a VCR!). The storage medium was fix at 20m on the reef wall. The recording period was 12h each day from 06:00 to 18:00. The reason for this is visibility. Outside these hours, it would not be possible to distinguish shapes on the film. That being said, this data does not account for nighttime activity. Only daytime activity.
So, what did they find? The most Hexanchus griseus activity on the reef (measured in inbound sharks per hour) occurred in the afternoon from mid-June to mid-July. No daytime activity was seen between October and May. None of the recorded H. griseus were mature, based on estimated TL from footage. Thus, this activity was not based on reproduction. No feeding activity was observed, either, despite the presence of known food items, such at Hydrolagus colliei, Squalus suckleyi, and Phoca vitulina. There's some ambiguity on the conclusion here. Just because this one camera angle didn't capture feeding doesn't mean feeding didn't occur. The likely reason for this pattern of diurnal activity, thus, is temperature regulation. The water temperature between 75 and 200m in the Strait of Georgia is about 8-9 degrees Celsius year round, whereas surface temps can get as cold as 6-7 in Winter and as high as 16-18 in the Summer.
This is really valuable information. If this can be confirmed, it could inform conservation efforts for Hexanchus griseus in the face of ocean temperature increase from climate change and global warming. The reef itself has seen a decline in biodiversity and fauna size over the last 150 years. This is altogether interesting, considering that Hexanchus griseus were thought to stay quite deep during the day, even in Summer.